Quantum rotation of Rb2(3Σu+) attached to HeN droplets: a path-integral Monte Carlo study

Doped HeNRb2((3)Sigma(+)(u)) systems, with N = 20 and 40, have been studied by means of a path integral Monte Carlo method at two different temperatures T = 1 K and 2 K. The impurity, Rb-2, is assumed as a rigid rotor and results are compared with a previous analysis in which no rotational or translational degrees of freedom were taken into account. Quantum effects are observed to play a noticeable role accounting for the extra energy with respect to the fixed Rb-2 case although differences between the two approaches do not seem to be as important as reported for some other dopants attached to helium droplets, such as OCS for example. Probability density distributions exhibit the same overall features as the non-rotating system, predicting the outer location of Rb-2 with respect to the helium atoms. The stability of the two clusters under study at T = 2 K is uncertain: the energy of He20Rb2 is positive and for He40Rb2, the observed dependence on the confinements imposed on the system precludes definitive statements regarding its physical existence.